In digital cellular mobile systems, speech communication is realized by transmitting and receiving the encoded speech information bit stream through a radio channel. Because of the possibility of transmission errors in the fading channel, the received data may be totally corrupted. Thus, a channel codec (coder-decoder) is essential to mitigate the effects of transmission errors.
The channel codec must carry out three fundamental functions:
a) error correction up to the capability of the channel coding employed; PA1 b) bad frame detection when the correction fails; and PA1 c) bad frame masking with the aid of the speech decoder to reconstruct the lost frame.
The error correction capability of a channel codec is determined by the error protection techniques used, (such as, convolution coding or Reed-Solomon coding) as well as other processing steps such as interleaving of frames, and feed-back of receiver status (soft channel coding).
In the new Digital Cellular Interim Standard 54 (IS-54), convolution coding and Viterbi decoding were selected as the error protection schemes.
The standard calls for a speech codec operating at 7950 bits/sec and able to produce 159 bits for each speech frame of 20 msec. The channel protection used in the standard is a multi-step channel codec. At the channel coder side, the 159 bits are regrouped into two classes of bits, according to their sensitivity to the regenerated speech quality. There are 77 class 1 bits and 82 class 2 bits. The class 1 bits are more sensitive than the class 2 bits.
A 7 bit CRC (Cyclic Redundancy Checking) is used for the purpose of error detection. These 7 bits are computed over the 12 most perceptually significant bits (this means that one error in these 12 bits will degrade severely the regenerated speech quality). These 7 bits are part of the class 1 bits. Thus, there are in total 84 class 1 bits.
An error protection technique based on convolutional coding is applied on the class 1 bits. This convolutional coding uses a rate 1/2, 5-bit memory convolutional code. The output bits from the convolution coder are then interleaved with the class 2 bits. Half of these bits are then transmitted at the current time slot and the remainder are transmitted 20 ms later.
The channel decoder operates in the reversed order. That is, the received data is de-interleaved in order to recover the bits corresponding to the current speech frame, and in order to separate these bits into class 2 bits and the bits to be convolutionally decoded. The received bits are then convolutionally decoded to recover the class 1 bits (error correction step) and a CRC check is done of the 12 most perceptually significant bits (error detection step). A bad frame masking technique is employed whenever the CRC check fails.
The problem with this approach is two-fold. First, the bad frame detection (CRC checking) is only based on the 12 most perceptually significant bits and the 7 CRC bits. Note that a single error in these 7 bits can also cause a CRC check failure. Errors which might have occurred in the other 65 class 1 bits are not taken into account. It was determined that even if the CRC check succeeds, there could be more than 45% bit error rate in these 65 bits.
Second, the bad frame masking repeats only the R0 and the LPC bits. The lag bits and the sub frame gain (GSP0) bits are always used as received. But, when CRC checking fails, the BER for these bits is usually very high (the average is around 25%).
In both cases, the use of the Lag bits and the GSP0 bits as received, having more than 10% bit errors, will degrade the regenerated speech quality. When errors occur in the GSP0 bits, the regenerated speech is usually explosion-like, which reduces considerably the intelligibility as well as the perceptual quality of the decoded speech.
Accordingly there is a need for an improved bad frame detection and masking technique which will help in the avoidance of explosion-like speech.
Thus, it is an object of the present invention to provide an improved error detection and bad frame masking technique which provides smoother regenerated speech, improves intelligibility and the perceptual quality of speech.
Another object of the present invention is to provide an improved error detection technique so that errors occurring in the Class 1 bits, other than the most significant bits, can be taken into account.
Another object of the present invention is to provide an improved bad frame masking technique so that erroneous information will not be used by the speech decoder
Yet another object of the present invention is to provide an improved error detection and bad frame masking technique which can be implemented with the requirements of the digital cellular standard.